1. Field of the Invention
The present invention generally relates to the cable modems and up-conversion in cable modem systems.
2. Background Art
In a cable modem system, a number of channels, occupying a frequency range from 50 megahertz (MHz) to 860 MHz, are transmitted from a cable head-in over a cable system to one or more users. A conventional cable system transmitter, or cable modem termination system (CMTS), is composed mainly of analog components. That is, a conventional CMTS transmitter primarily comprises a combination of mixers, filters, amplifiers, and summation circuits in order to transmit and process a base-band signal. The base-band signal must then be modulated up to the required frequency, which is typically from about 50 to 860 MHz.
Also, in conventional CMTSs, a transmit up-converter within the CMTS utilizes an analog phase lock loop (PLL) for timing purposes. The PLL will typically include multiple clock sources, or a single tunable source, to generate local oscillator frequencies for up-conversion of the cable channels. This approach, however, creates multiple signals that can contribute to signal interference and distortion.
As the level of performance increases in the conventional CMTSs, testability also becomes an issue. For example, in the exemplary case of a 12-bit digital to analog converter (DAC), used within the CMTS, the performance level is commensurate with clock frequencies in excess of 2 giga-samples per second. In the absence of special test measures and procedures, it is virtually impossible to test, for example, this 12-bit DAC while the DAC is running as fast as giga-samples per second.
What is needed therefore is a system and method to provide modulation and block up-conversion to higher frequencies in the digital domain. It is desirable that elements of an up-converter within this system be formed on a single integrated circuit (IC). What is also needed is a system and method providing a built-in test capability for the system noted above.